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25 kA, 31,5 kA and 40 kA are typical ratings of switchgear with respect to maximum short-circuit current capabilities (typically denoted \$I''_k\$)

I can understand 40 and 25 as they are nice numbers - but 31.5 is downright ugly. How has that number become an standard ? Does anyone know the history or background here?

fluxmodel
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It's part of a power series. To a close approximation, 25 x 1,25 = 31,5. Then 31,5 x 1,25 = 40. The next few values are 50 and 63, which also appear a lot in component specifications.

Simon B
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  • I'll accept that. Thanks. – fluxmodel May 31 '21 at 11:50
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    Related: [why-is-the-number-63-popular-in-electronics](https://electronics.stackexchange.com/questions/564511/why-is-the-number-63-popular-in-electronics/564523#564523). It's part of the R10 geometric series with ten steps per decade. – Transistor May 31 '21 at 13:25